Stereochemical rigidity in ML5 complexes. II. Preparations and

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 27, 2015 | http://pubs.acs.org Publication Date: September 1, 1974 | doi: 10.1021/ja00825a011

5751

an enolate intermediate to which a proton is added. It would be presumed that this protonation would result in racemization in the absence of any steric hindrance. An inspection of Figure 5 shows that there is no steric hindrance to the approach of a proton to C(10), although experimental results indicate a proton adds preferentially from the top of this figure, inverting the configuration. It might be thought that either O(3) of the decarboxylation product (COz) is slow to leave (compared to the time for protonation) or that another molecule, perhaps water, is hydrogen bonded to N(3) at this site, partially blocking rapid protonation. Figure 5 shows that pathway of a proton to give inversion of configuration is less obstructed. E. Summary. In conclusion we have found ( I ) that three-point attachment provides a sufficient condition for chiral recognition of "equivalent" groups in a symmetrical, prochiral molecule, (2) that the product distribution upon addition of a,&-aminomethylmalonate (or any a-amino acid) to a Co(1II) tetramine can be explained by invoking a trans intermediate, and (3) that resolution of C D spectra into component Cotton

effects can provide useful information as to the chirality of asymmetric centers induced on complexation. The fact that complexing to an agent with two open binding sites may derive stereospecific recognition via the presence of a third, hydrogen bonding, site opens new avenues for the creative design of template molecules. Acknowledgments. We are grateful to Mr. Paul R. Hansen and Miss Barbara Gallen for technical assistance. This research was supported by Grants CA10925, CA-06927, RR-05539, and AM-09171 from the National Institutes of Health, U. S. Public Health Service, and by an appropriation from the Commonwealth of Pennsylvania. Supplementary Material Available. A listing of structure factor amplitudes for the a,a-aminomethylmalonate complex will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this naper only or microfiche (105 X 148 mm, 2 4 X reduction, negarives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for $4.00 for photocopy or $2.00 for microfiche, referring to code number JACS-745741.

Stereochemical Rigidity in ML, Complexes. 11. Preparations and Intermolecular Exchange of Cationic ML, Complexes of Cobalt(I), Rhodium(I), Iridium(I), Nickel(II), Palladium( II), and Platinum( 11) P. Meakin" and J. P. Jesson Contribution No. 2120 f r o m the Central Research Department, E. I . du Pont de Nemours & Company, Experimental Station, Wilmington, Delaware 19898. Received December 8, 1973

Abstract: A series of complexes of the form ML5"+(X-), have been synthesized (M = Co(I), Rh(I), Ir(I), Ni(II), Pd(II), Pt(I1); L = phosphite). The Pd(I1) and Pt(I1) species are novel in that they represent the first ML5 compounds for these metals; complexes in this class have been described previously for the other four metals. We report new preparative techniques for Co(1) and Ir(1) and extend the scope of known complexes for several of the metals. The steric range within which MLj species may be prepared is defined in connection with the equilibrium ML5 e ML4 L and the steric size of L. Intermolecular exchange effects associated with the above equilibrium are analyzed. Preparative problems associated with transesterificationare discussed.

+

T

he question of stereochemistry and fluxional behavior in five-coordinate transition metal complexes has been a problem of considerable interest since the early nmr experiments of Cotton, et al.' Recently we have reported the first nmr evidence for stereochemical rigidity and trigonal bipyramidal equilibrium stereochemistry for complexes of the form RhL,+X- (L = together with a detailed line shape analysis indicating that the permutational nature of the exchange process2 is consistent with the Berry mechanism.4 As a continuing part of this investigation a substantial ( I ) F. A. Cotton, A. Danti, J. S. Waugh, and R. W. Fessenden, J . Chem. Phys., 29,1427 (1958). (2) J. P. Jessonand P. Meakin, J. Amer. Chem. Soc., 95, 1344(1973). (3) P. Meakin and J. P. Jesson, J. Amer. Chem. Soc., 95,7272 (1973). (4) R. S. Berry, J. Chem. Phys., 32,933 (1960).

number of ML, complexes of Co(I), Rh(I), Ir(I), Ni(II), Pd(II), and Pt(I1) have been prepared; some preliminary results with regard to stereochemistry and stereochemical rigidity in these complexes have been communicated., The present paper describes the preparation of the MLjn+(X-), complexes together with an analysis of intermolecular exchange in Rh[P(OCH3)3],+B(C,H5)4-. Prior to this work, the only well-established fivecoordinate species of the type ML, for platinum was Pt(SnC13),3-;6 there appeared to be no well-characterized ML: species for Pd. The PtLj2+ and PdLj2+ phosphite complexes described here are therefore signif(5) J. P. Jesson and P. Meakin, Inorg. Nucl. Chem. Lett., 9, 1221 (1973). (6) R . D. Cramer, R. V. Lindsey, Jr., C. T. Prewitt, and U. G . Stolberg, J . Amer. Chem. Soc., 81,658 (1965).

Meakin, Jesson

Stereochemical Rigidity in ML5 Complexes

5752

44 SPECTRA OBSERVED

Rh[P(OCH,),]J CALCULATED

OBSERVED


CALCULATED

J

I-IM'I iJi -120 6

B(C&);

31P['H I 36.43 MHz

-35

Figure 1. The 31P{IH) spectra for a variety of MLa complexes together with simulations using an A2Ba model. The spin-spin coupling constants are in hertz and the chemical shift differences are in hertz at 36.43 MHz.

icant extensions of the known scope of five-coordination. Transesterification reactions occur rather easily with most of the compounds and can cause preparative problems since the effects are not always apparent from chemical analysis. 31Pnmr provides a clear indication of when such side reactions have occurred. Results Stereochemistries of ML5 Complexes. The slowing down of the intramolecular exchange allows one to determine unambiguously for the first time the stereochemistry of a class of ML5 species in solution. Two idealized geometries (the trigonal bipyramid 1 and the square pyramid 2) have been established for ML5species A

1

2

in the solid state using X-ray techniques. Configuration 1 would give rise in the slow exchange limit to A2B3 'PI 'H ] patterns for X = Co, Ir, Ni, and Pd, to A2B3X patterns for X = Rh (lo3Rh is 100% abundant with Z = I/?), and to a superposition of A2B3(67% of the intensity) and A2B3X(33z of the intensity) for X = Pt (lg5Ptis 33% abundant). Configuration 2 would give AB4 patterns for X = Co, Ir, Ni, and Pd, to AB4X patterns for X = Rh and to a combination of AB4 and ABiX patterns for X = Pt. For both cases, the limiting fast exchange spectra would be As or A5X patterns. Journal of the American Chemical Society 1 96:18

Figure 2. Observed and calculated 31P{'H) nmr spectra for a solution of Rh[P(OCH3)3]5+B(CeH5))r(0.125 M ) in acetonitrile over the temperature range for which intermolecular exchange effects can be observed.

For all the molecules studied in the present paper slow exchange limit spectra have been observed and they all correspond to the trigonal bipyramidal structure 1. A series of low temperature limit spectra encompassing a wide range of J/S are shown in Figure 1 together with simulations using an A2B3 model. It seems likely that the trigonal bipyramidal geometry will prevail for most d3 ML5 complexes in solution and that deviations from this stereochemistry in the solid state can often be ascribed t o crystal packing effects. It should be remembered that the energy difference between configurations l and 2 is commonly less than 4 kcal mol-' (assuming that intramolecular exchange takes place via the Berry process4 in which case 1 is the ground state and 2 the transition state). Rapid intramolecular exchange occurs for all the MLs complexes at higher temperatures, and the expected A5 or A5X fast exchange limit spectra are observed. At still higher temperatures ligand dissociation (eq 1) causes a collapse of the A5X doublets into a single

September 4, 1974

5753 Table I. Chemical Shifts and Coupling Constants for MLs Complexes at the Temperatures Noted 6A"

Complex

6 B"

IJABI

-5.0 -10.4 11.9

148 148 147

JAX

JBX

Temp, "C


co+ P(OCHd3 P(0CzH 513 P(OCHz)3CCHa Rh+ P(OCHds P(OCzH 5 1 3 P(OCIHI))B P(OCH2)3CCH3 P(OCHz)3CCzHj P(OCHCH2)a Ir+ P(OCHd3 p(OczHd3 P(OCaH& P(OCHz)3CCH3 Ni2+ P(OCH3)a P(0CHz)aCCHS P(OCHz)3CCzHj P(0CHCHz)a Pd2+ P(OCHd3 P(OCzH 513 P(OCHz)aCCH3 Pt2+ P(OCHd3 P(OCzH:)3 P(OCH2)3CCH3 a

-11.5 -4.3 -8.6

-138 -138 - 122

11 .o 18.1 30.8 14.8 13.6 -5.0

-3.7 -0.6 18.9 25.4 24.3 -0.9

68 68 70 69.5 69.5 69.5

55.8 64.4 61.4 54.0

26.9 34.1 43.6 57.2

59 60 59.5 59

28.5 21.4 23.1 9.7

29.4 36.8 38.7 17.1

120.0 121 .o 119.5 117.0

-120 -123 - 125 - 109

40.2 47.7 40.6

30.4 38.2 47.2

48.5 51 .O ~50.0

- 157 - 146

79.8 81.9 75.4

58.6 63.4 72.0

52.5 47.0 52.0

i143 =!= 143 =!= 142 1140 ?L 140 ?L 141

i206 i205

i205 i208 =!= 208 i207.5

-136 -131 -113 - 127 -137 -92 -152

- 130 - 77 - 109

- 160 i2840 rt2845 i2744

=l=4100 14060 zk4123

- 140 -130 -135

Parts per million upfield from P(OCH3)3in CHClFz.

ML,

e MLa + L

line due to loss of coupling in the bond breaking process or a broadening of the A; spectra in the presence of added ligand due to an averaging of free and bound ligand resonances. The chemical shifts and coupling constants for the 31P{'H 1 spectra of the ML;, species are given in Table I. It should be noted that in some cases the chemical shifts and chemical shift separations are markedly temperature dependent3 so that comparisons of the parameters can only be made at the temperatures noted. The coupling constants are insensitive to temperature variation. Intermolecular Exchange in the Rh[P(OCH3)3]j+Rh[P(OCH3)3]4+-P(OCH3)3System. The 31P('H ] nmr spectrum for solutions of Rh[P(OCH3)3]3-B(C6H,)4in acetonitrile near room temperature consists of a sharp doublet (J(31P-103Rh)= 182 Hz). As the temperature is raised, the doublet begins to broaden and eventually collapses into a sharp single line (Figure 2). The loss of the ro3Rh-31Pcoupling in the high temperature limit is characteristic of an intermolecular process involving breaking of the phosphorus-rhodium bonds. 'H Figure 3 shows the temperature-dependent nmr spectra for a 0.125 A4 solution of Rh[P(OCH,)],+B(C6H6)4-in CH3CN to which 0.5 mol 1.-' of P(OCH3)3 has been added. Since addition of free ligand does not markedly accelerate the rate of exchange, the process must be dissociative (eq 2 ) with the equilibrium far to the left.

solution. Since only one out of five possible ligands dissociates in a single dissociation step, and since 50z of these will return to a RhL4+cation with the same component of Io3Rhspin angular momentum as that from which it dissociated, the nmr line shapes can be obtained using the two-site (one spin) model shown in Figure 4. (At each site four-fifths of the ligands do not dissociate in a single event thus retaining their coupling to the lo3Rh spin and one-tenth of the ligands return to a Io3Rhspin with the original orientation for a total of 4 / 5 = 9/10; only one-tenth of the total appears at the site with opposite orientation of the Io3Rhspin.) Similarly, the nmr line shapes for the RhLj+-L system shown in Figure 3 can be calculated using a threesite model based on eq 2. This three-site (one spin) model is shown in Figure 5 and the simulated spectra obtained are shown on the right-hand side of Figure 3. (At each of the two possible Rh sites four-fifths of the ligands do not dissociate in a single event thus retaining their coupling to the Io3Rh spin and one-fifth of the ligands join the pool of free ligand in solution.) For both Figures 2 and 3 the observed and calculated spectra are in very good agreement and the calculated values of the rate constant kl are quite close at any particular temperature. Similar calculations were also carried out for a solution with 0.25 mol 1.-' of added P(OCH3)3. From the nmr line shape analysis of the RhLj+ solutions with 0, 0.25 and 0.5 mol 1.-' of P(OCH&, we obtain the Arrhenius rate expression k,(T) = 1014.6e-l6,5OO/RT

The calculated spectra shown in the right-hand column of Figure 2 were obtained assuming that the concentrations of RhLA+ and L are negligible in the RhLj+

or in terms of the Eyring equation AG*300 = 14.0 kcal mol-', AH'300 = 15.9 kcal mol-', and AS*aoo = 6.2 cal mol-' deg-1. The small positive entropy of activation is consistent with a dissociative process. Figure 6 shows the observed 31P{'H ] nmr spectra for

(1)

+

Meakin, Jesson 1 Stereochemical Rigidity in MLj Complexes


5754

Figure 4. Exchange diagram for two-site (one spin) model used to calculate intermolecular exchange spectra for RhLs+ solutions (see Figure 2).

RhL5+

Figure 3. Observed and calculated 31P('H1 nmr spectra for a solution containing Rh[P(OCH3)31j+B(C6He)(- (0.125 M ) and P(OCH3)3 (0.5 M ) in acetonitrile over the temperature range for which intermolecular exchange effects can be observed.

two RhLj+-RhL4+ solutions as a function of temperature. Again, the total rhodium concentration is 0.125 mol I.-' with C H K N as the solvent. These spectra have been analyzed in terms of the exchange process given in eq 2 using the four-site (one spin) model shown in Figure 7. It is assumed that the free ligand concentration is negligibly small. (At either of the two RhLj+ sites a single dissociation process will put the remaining undissociated four-fifths fraction of ligands into one of the two RhL4+ sites corresponding to an unchanged z component for the Io3Rhspin; the one-fifth fraction of dissociated ligand has an equal probability of returning to either of the RhL5+ sites (one-tenth to each).) Calculations were carried out for the two different [RhLjt] :[RhL4+]ratios(w2 : 1 and -1 :2). Reasonable agreement between the calculated and observed spectra was obtained although not as good as that shown in Figures 2 and 3. The values for the rate constant kl were similar to those from the analyses of the RhLj+ and RhLj+-L spectra. The good agreement between the rates obtained for

5 Figure 5. Exchange diagram for three-site (one spin) model used to calculate intermolecular exchange spectra for RhLs+-L solutions (see Figure 3). Only the forward reaction path is shown in the diagram. The reverse path must, of course, be included in setting up the exchange matrix from the exchange diagram.

the RhL4+-RhLj+, the RhLjf and the RhL5+-L systems at all temperatures in the range 27-85' is taken to support the validity of the model used (eq 2) and the assumption that the degree of dissociation of RhLj+ is small in acetonitrile over this temperature range. Since the MLj+ complex is undergoing very rapid intramolecular rearrangement in the temperature range 0-85 O , it is not possible to tell if it is an axial or an equatorial ligand which dissociates. It should be noted that the broadening of the RhL5+ resonances on adding free ligand in the slow-medium exchange region (compare particularly the 27 O spectra in Figures 2 and 3) is not a result of a change in the preexchange lifetime ( I l k l ) of the RhLj+ cation. If the RhLj+ cation is present in solution without significant RhL4+ and L concentrations the effective preexchange lifetime for a line in the RhLj+ spectrum is 10/kl (uide supra and Figure 4). If free ligand is added to the

Journal of the American Chemical Society 1 96:18 1 September 4 , 1974

5755


I'

&-

Figure 7. Exchange diagram for four-site (one spin) model used to calculate intermolecular exchange spectra for RhLj+-RhLd+ solutions. Again only the effects of the forward part of the reaction are shown. The RhL4+doublet is to the right side of the R h L 2 doublet.

RhLiIL k,=500SEC-'

n

4

Figure 6. Observed 31P(1H\ nmr spectra for two solutions of RhL2+-RhL4* mixtures in acetonitrile over the temperature range where intermolecular exchange effects can be observed. The lefthand column is for a solution with 63 % RhLj+ and the right-hand column is for a solution with 32z RhLe+.

solution, one-fifth of the ligands in a RhL5+ ion exchange with the free ligand pool in a single dissociation and the effective preexchange lifetime for a line in a RhL6+ spectrum is 5/k,,provided the concentration of added ligand is sufficiently large. Consequently, the addition of appreciable concentrations of free ligand should effectively double the width of the RhLj+ resonances even though there is no change in the true preexchange lifetime (l/kl). This result has been confirmed by repeating the line shape calculations shown in Figure 2 using the model shown in Figure 5 with a very low free ligand concentration. The left-hand side of Figure 8 shows a simulation of the RhL6+-L system as a function of added ligand concentration using the model shown in Figure 5, keeping k, fixed at 500 sec-' and varying [L]. These calcula-

Figure 8. The left-hand side shows a simulation for the RhL6+-L system as a function of [L]/[RhL:+] keeping kl fixed at 500 sec-l. The calculations on the right-hand side are for RhLi'-RhL,+ mixtures with kl fixed at 100 sec-'. A large transverse relaxation time was used in these calculations; consequently, only the exchange contributions to the line widths are shown. The models shown in Figures 5 and 8 were used for the left and right columns, respectively.

tions show that as the free ligand concentration is lowered the widths of the two lines assigned mainly to the RhL6+ cation decrease until, at a sufficiently small free ligand concentration, a limiting value of half of that calculated in the presence of large [L] is obtained. The 2 : 1 ratio of the line widths in the limit of high and Meakin, Jesson 1 Stereochemical Ridigity in M L Ciunplexes

5756

and co(1). While this paper was in preparation complexes of Ir(1) were reported'5 using a different synthetic route. Cobalt(1). The earliest preparative method involved a disproportionation reaction of Co2+in the presence of phosphite ligandl0-l3 (eq 3). More recently a route 2Co*+

+ 11L +CoL,+ + C0L63+

(3)

involving photolysis of CO(CO)[P(OCH~)~]~+ (obtained from C O ~ ( C O ) ~ excess P(OCH3)3)in the presence of P(OCH3)3has been described. l 4 We have developed a method starting from Co(C8HI2)(C8H13)which is simple and quite general. Co(C8HI2)(C8HI3) itself is, however, quite difficult to prepare; l 6 starting from this compound near quantitative yields are obtained by the following reaction sequence carried out in alcoholic solution. Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 27, 2015 | http://pubs.acs.org Publication Date: September 1, 1974 | doi: 10.1021/ja00825a011

+

Figure 9. Two attempts to prepare Rh[P(OCgH&]s+B(CsH5)4-: (A) in methanolic solution, the product has undergone partial transesterification; (B) in ethanolic solution, the correct product is obtained.

low [L] can be established quantitatively from the spectral v e c t ~ r . ? ~ ~ The effect can be seen even more dramatically by comparing the 27" spectra in Figures 2 and 6 . In the presence of RhL4+each dissociation step puts four-fifths of the 31Pnuclei in a RhL5+cation into the RhL1+ pool and transfers one-tenth of the RhLj+ 31Pnuclei to a site at which the Io3Rhspin has changed orientation (uiu a free ligand site present in vanishingly small concentration). Consequently, the effective preexchange lifetime for a line in the RhL5+spectrum is (k1/10+ 4k1/5)-' = 10/9k1 (Figure 7 ) and addition of RhL4+to the RhL5+solution will increase the RhLj+ line width by a factor of 9 (again there is no change in the true preexchange lifetime l/kl of the MLj+ cation). The above considerations show that a superficial analysis of the spectra shown in Figures 2, 3, and 6 (e.g., comparing line widths at a given temperature) could lead to the erroneous conclusions that addition of either free ligand or RhLI+ to a solution of RhL5+ shortens the lifetime of a RhLj+ cation, that the dissociation is not first order, and that the mechanism implied by eq 1 is not consistent with all the data presented. The right-hand side of Figure 8 shows a simulation for the RhL5+-RhL4+ system as a function of [RhL4+] using the model shown in Figure 7 . In this calculation, kl was kept fixed at 100 sec-I and [RhL4+]was varied. An examination of the spectral vectors? in the limits of high and low RhL4+ concentration confirms the 9 : 1 ratio of line widths. Preparation Methods for ML5 Complexes. At the time when this work was completed, M L j cations had been described in the literature for Rh(I),g Ni(II),'O-13 (7) G . Binsch,J. Amer. Chem. Soc., 91,1304(1969). (8) We have observed that on addition of increasing amounts of free ligand to a solution of Rh[P(OCH3)~]s+B(CaHs)~in C H E N at 27", the RhL5+ line width at first increases and then levels off at twice the line width observed in the absence of added ligand; this is exactly the behavior predicted by the calculations assuming a dissociative mechanism. (9) L. M. Haines,Inorg. Chem., 10, 1685(1971). (10) J. G. Verkade andT. S. Piper, Inorg. Chem., 2,944(1963). (11) T. J. Huttemann, B. Foxman, C. Sperati, and J. G . Verkade, Inorg. Chem., 4.950 (1965). (12) K . J. Coskran, T. J. Huttemann, and J. G . Verkade, Aduan. Chem. Ser., No. 62,590 (1966). (1 3) J. G. Verkade and K. J. Coskran in "Organic Phosphorus Compounds," Vol. 2, G . M. Kosolapoff and L. Maier Ed., Wiley, New York, N. Y.,1972, Chapter 3B.

Journal of the American Chemical Society

1 96:18 1 September

L,Co(CsHi,) coLj+c1-

CHaCOCl

CoL:+CI-

Na + B ( C B H ~ ) ~ -

+COLj+B(CeH5)?-

The initial addition of excess ligand displaces the cyclooctadiene, subsequent addition of 1 equiv of acetyl chloride generates 1 equiv of HCl by reaction with the solvent alcohol, and the HC1 cleaves the Co-C bond in L,Co(CsHI3). These phases of the reaction sequence are carried out without isolation of any of the products and result in a clear alcoholic solution from which C O L ~ + B ( C ~ H can ~ ) ~ -be precipitated quantitatively by addition of sodium tetraphenylborate. In situ preparations in nmr tubes could be carried out for all six metals, in the absence of alcohol (to avoid transesterification), using CH2CI2,C H X N , or CHCIFz as solvents and, for Co, CF3COOH to cleave the Co-C bond. Rhodium(1). RhL6+ phosphite species were first synthesized from [(CSHl2)RhC1],by addition of excess ligand to an alcoholic solution and precipitation with B(C6H& or P F O - . ~ Our procedure is similar starting from [(C2H4)2RhC1]2;it was found that if the preparation was carried out in an alcohol which did not correspond to the phosphite ligand being used (e.g., if methanol were used as the solvent for the preparation of Rh[P(OC2H5)3]5+)the clean A2B3X 31P('H) spectra described in the first paper in this series were not always observed. A comparison of the spectrum for the solids obtained from an attempted preparation of Rh[P(OC2Hj)3]5+in methanol (Figure 9A) and the spectrum for the same preparation in ethanol (Figure 9B) is shown in Figure 9. It is clear that appreciable transesterification has occurred in the former case and that a clean product is obtained in the latter case. In addition, it was found that, while preparations of tetraphenylborate salts were uniformly successful if the correct alcohol was used, attempts to precipitate hexafluorophosphate salts often led to solids which did not give the correct 31P('HI spectra. Jt is possible that problems of the type discussed above could pass undetected if normal chemical analysis were used for characterization. (14) S. Attali and R. Poilblanc, Inorg. Chim. Acra, 6,475 (1972). (15) L. M. Haines and E. Singleton, J . Chem. Soc., Dalton Trans.,

1891(1972). (16) The preparative details were worked out by Dr. L. W. Gosser and Mr. M. A. Cushing (to be submitted for publication).

4, 1974


5151

Iridium(1). Although RhLj+ species can be prepared by reaction of excess ligand with [(C8H12)RhC1]2at room t e m p e r a t ~ r e the , ~ corresponding reaction for Ir(1) does not go to completion under these conditions. Several runs were made using [(C8H12)IrC1]2and in all cases, using 31P{1H 1 nmr as the analytical tool, no appreciable amounts of IrL5+ were detected. Rather than running at higher temperatures, which can apparently give satisfactory results,14 we looked for a starting complex in which the ligands were more easily displaced than cyclooctadiene. On the assumption that the chelation effect contributes significantly to the stability of [(C8Hlz)IrCI],, a complex with nonchelating olefin ligands, [(C8H11)21rC1]2, was synthesized ; the cyclooctene groups are easily displaced by phosphites at room temperature. The procedures were similar to those used for Rh(1) and are described in the Experimental Section. Nickel(I1). The starting material of choice was Ni(BF4), 6HzO; experiments with Ni(N03)2 6Hz0, for instance, usually proved unsuccessful due possibly to the ability of the nitrate ion to coordinate to the central metal. Ni(BF4)?.6H20is dissolved in alcohol in the presence of excess ligand; NiL52+(BF4-)2salts are normally more soluble in alcohol than are the salts of heavier anions such as SbC16- or B(C6H&- so that compounds containing the heavier ions can readily be obtained by precipitation. Tetrafluoroborate salts can be obtained by concentrating and chilling the alcoholic solutions. Palladium(I1). Although PdLb2+ species have not previously been prepared, the synthetic approach used in this paper is quite straightforward. An alcoholic suspension of PdCI, is stirred in the presence of excess phosphite ligand until the major portion of the solids dissolve. The solution is filtered and a precipitating agent such as Na+B[C6HjI4- is added. The only precaution which must be observed is to avoid loss of the fifth ligand in PdLS2+during the recrystallization and washing processes since ligand dissocation (eq 7) occurs

-

+

e PdLdZ+ + L

PdLj'J+

(7)

at an appreciable rate at room temperature. The simplest technique for suppressing this decomposition reaction is to wash with solvent containing appreciable amounts of phosphite and to recrystallize slowly from chilled solutions containing some excess phosphite. Platinum(I1). The preparative procedures are precisely analogous to those described in the previous section for Pd2+. Single crystals of the salts Pt[P(OCH &]S**(PFG)Z and Pt[ P(0CH *+[B(C6H &-]a were grown by slow evaporation, and a preliminary Xray examination was undertaken. The hexafluorophosphate salt gives monoclinic crystals with the space groupoP21/C; the owit cell ha? the parameters a = 10.07 A, b = 11.89 A , c = 37.34 A , a n d p = 105.7". Transesterification of Phosphites. Transesterification of free phosphites with alcohols can in some cases occur rapidly; evidence has been presented to suggest that coordinated phosphites transesterify much less easily. l8 (17) F. W. Hoffmann, R. J. Ess, and R. P. Usinger, Jr., J. Amer. Chem. Soc., 78, 5817 (1956), detected transesterification in P(OR)a -b

In cases where the alcohol corresponding to the phosphite of choice is a good solvent, this preparative problem can be avoided by using the appropriate alcohol as solvent. In other cases where the alcohol is not a suitable solvent, the preparations can sometimes be carried out successfully in other solvents such as THF. In still other cases the transesterification reaction is sufficiently slow that good preparations can be carried out using an alcohol solvent that does not correspond to the phosphite; here it is important to work close to room temperature. In reactions of this type which were carried out under reflux transesterification were always extensive and often complete. A room temperature preparation of Ir[P(OC2Hj)3]j+B(C6Hj)~in methanolic solution gave the correct product both by elemental analysis and 31P(1H] nmr. On the other hand, an attempted preparation of Pt[P(O-n-C4HS)3]5*+[B(C6H&-I2 in ethanal under refluxing conditions gave P~[P(OCZH&]~*+[B(C~H&-]~ quantitatively. Chemical Shifts and Coupling Constants. The absolute values of the 31Pshift separations for constant ligand increase on going down a vertical triad and are uniformly higher for the doubly charged metal relative to the corresponding singly charged metal, an effect which has previously been observed in other isoelectronic isostructural series. l 9 The complex Ni[P(OCHCH2)8]62+(C104-)z is the only MLa species of the type we are considering for which an X-ray crystal structure determination has been made. *O The cation is an accurate trigonal bipyramid with the axial bond lengths measurably shorter than the equatorial bond lengths. It may be noted from Table I that for this cation the phosphorus resonances associated with the equatorial ligands are to high field of those associated with the axial ligands and also for a given metal the sign of the axial-equatorial 31Pshift difference can be different for different ligands; it would be of interest on the basis of suitably chosen cases to compare shift differences with ratios of axial to equatorial bond lengths for other complexes. The phosphorus-phosphorus coupling constants are remarkably constant for a given metal and are much bigger for the first-row metals than for the remainder (-150 Hz for Co+, -120 Hz for Ni2+). The values for the four second- and third-row metals lie in a narrow range ( 4 0 - 7 0 Hz). Similar constancy is found for the Ma,-P and Mo,-P couplings for Rh+ and Pt *+. For both metals J,, > J,, and the ratio Jax/Joq= 0.7 despite the large change in absolute magnitudes ; if the mechanism for transmission of spin correlation is primarily Fermi contact, the ratio would roughly reflect the ratio of the s characters for the two types of bond with the equatorial bonds having appreciably greater s character. Experimental Section 31P ( 1H 1 Fourier mode nmr spectra were recorded as described in the first publication in this series.' Methods of preparation of the compounds are described in the appropriate sections below. I. Cobalt Complexes. A. C O ( C ~ H ~ ~ ) ( CPrecursor. ~ H ~ ~ ) The has been preparation of r-cyclooctenyl-a-cycloocta-l,5-dienecobalt

R'OH solutions by heating the mixture to remove ROH reaction product. (18) D. H. Gerlach, W. G. Peet, and E. L. Muetterties, J. Amer.

(19) I50 of the intensity) assigned to Rh[P(OCH3)3]4+PF6-. (b) Successful iri situ preparations were carried out in nmr tubes; to 0.5 ml of CH2C12in a 10-mm nmr tube at room temperature were added 97.3 mg of [(C2H,)2RhC1]2,290 pl of P(OCH3)?,and 81.5 mg (21) H . Lehmkuhl. W. Leuchte, and E. Janssen, J . Organometa[. Chem., 30,407 (1971). (22) S . Otsuka and M. Rossi,J. Chem. SOC.A , 2630(1968). (23) (a) R . D. Cramer, Inorg. Chem., 1, 722 (1962); (b) Itiorg. Syn., in press.

Journal of the American Chemical Society

of NH4PF6. After ethylene evolution was complete, the tube was cooled in Dry Ice and the total volume was raised to 4 ml by The tube was cooled to -70" and addition of CHCIFz (bp -40"). shaken to ensure complete mixing. The 31P(' H ) nmr spectrum was examined at low temperature revealing the characteristic A2B3X patl.ern assigned to Rh[P(OCH3)a]5+PF6-together with a signal assigned to the excess P(OCH& Compounds iii-vi were prepared in a similar manner. (iii) Rh[P(OCH3)3]j+AsF6-. (iv) Rh[P(OCH3):] s+SbF6-. (v) { Rh[P(OCH3)2]j+]zSiF62-. (vi) ( Rh[P(OCH3)&+ )&eFs2-. ~ H ~compound )~-. was prepared (vii) R ~ [ P ( O C Z H ~ ) ~ ] ~ + B ( CThe successfully in a manner similar to (i) using methanol as solvent. Preparations in methanol gave partial transesterification. $lP(lH ] spectra are compared in Figure 9. (viii) Rh[P(O-n-C4H&15+B(C6Hs)4-. Procedures analogous to (i) with rz-butyl alcohol as solvent and analogous to (ii) (b) with CHICIZ-CHCIR as solvent were successful. Attempts using methanol as solvent gave products with some transesterification (determined by 31P( 1H 1 nmr). (ix) R~\P(OCHZ)~CCH~]~+B(CJ€:)~-. A method similar to (i), was employed using methanol 2 s solvent at room temperature. ~ ) , -complex . was pre(x) R ~ [ P ( O C H Z ) ~ C C Z H J ~ + B ( C ~This pared in situ in a manner similar to that described under (ii) (b). (xi) Rh[P(OCHCH2)3ljfB(CsHj)4-.The 2,8,9-trioxa-l-phosphaadamatane complex was also prepared iii situ from a small quantity of ligand kindly supplied by Professor J. G. Verkade. The 31P{ 'H 1 nmr spectrum js shown in Figure 10 together with a simulation using an AzBzXmodel. (xii) Rh[P(OCH3),I4+B(C6Hj)-. A 500-ml erlenmeyer flask was charged with 200 ml of nitrogen-saturated methanol. [(C2H&RhC112 (1.94 g, 0.005 mol) was added followed by P(OCH:J3(4.97 g, 0.040 mol) ; the mixture was stirred until ethylene evolution ceased and all the solids were dissolved. A solution of NaB(CsH& (3.44 g, 0.01 mol) in 30 ml of methanol was introduced slowly. The mixture was partially stripped, filtered, and dried. 31P('H ] studies showed the presence of only a trace of RhLi with the desired RhL4+. This compound was used for the intermolecular exchange studies involving RhL,+. 111. Iridium(1). A. [(C8Hlr)21rC1]2 Precursor. The procedure follows closely that described by van der Ent and Onderdelinden.21 B. Salts. (i) Ir[P(OCH3)3]j+B(C6H:,)4-.A 125-ml d e n meyer flask was charged with 25 ml of methanol and [(CiH14)21rCI]2 (0.90 g, 0.001 mol). P(OCH3), (1.66 g, 0.013 mol) was added to the slurry over a period of 2 min and stirring was continued until all went into solution. Sodium tetraphenylborate (0.83 g, 0.0024 mol) was dissolved in approximately 5 ml of methanol and added slowly, dropwise, to the solution to precipitate off-white crystals. The crystals were collected on a mediumfritted disk funnel and dried iri caciiu for 20 min. The complex was also prepared by irz situ procedures similar to those described under (iv) below. (ii) Ir[P(OCH3),lj+PF6-. Attempts to prepare this complex by a procedure similar to that described for the corresponding rhodium complex in section I1 B (ii) led to similar results, i.e., products which were >50% contaminated with IrL4' as shown by 31P(' H } nmr measurements. The compound was prepared (iii) Ir[P(OC?Hs)3]j+B(CsH5)4-. and isolated in a manner similar to 111B (i) and was prepared in situ by procedures similar to those described under (iv) below. (iv) I~[P(O-/Z-C,H~)~]~+B(C~H~)(-. The complex was prepared b i situ in a 10-mm nmr tube by adding 224 mg of [(CsHlr)21rC11?, 625 mg of P(O-rz-C4Ho),,and 171 mg of NaB(C6H& to 1 ml of CHZCIS. After a short period the tube was cooled in Dry Ice and the total volume was raised to 4 ml by addition of CHCIF2. The tube was cooled to - 80" and shaken to ensure complete mixing. (v) I ~ [ P ( O C H Z ) ~ C C H ~ ] ~ + B ( C[(CsH14)zIrC1]2 ~H)~-. (1.5 g, 0.0016 mol), P(OCH&CCH3 (3.0 g, 0.02 mol), and NaB(CF;H& (1.4 g, 0.004 mol) were added to 75 ml of T H F in a 125-ml erlenmeyer flask equipped with a Teflon-coated magnetic stirrer. The mixture was refluxed for 5 min and stirred overnight, and the solids were collected on a medium frit and dried in cocuo. IV. Nickel Complexes. The precursor for all NiLj2+ preparations was commercial Ni[BF& 6Hz0.

(24) A. van der Ent and A. L. Onderdelinden, Inorg. Sjjn., 14, 9 2 (1973).

96:18 1 September 4 , 1974


5759 (i) Ni[P(OCH3)3]:2+ (BFa'-)2. A 250-ml erlenmeyer flask was charged with 200 ml of methanol and nickel tetrafluoroborate (3.00 g, 0.0088 mol). After stirring 10 min, all went into solution, and trimethyl phosphite (15.00 g, 0.12 mol) was added dropwise to the green solution, which, upon addition of ligand, changed to orange-red. A 50-ml portion of the solution was partially stripped and chilled (-40") under nitrogen overnight. White needle-like crystals were formed and collected on a medium-fritted disk funnel. The product was dried itz C ~ C U Ofor 2 hr. The remaining 200 ml of solution from the first part of this reaction was used in 50 ml portions for the preparation of compounds ii-v. In each case, the appropriate anion was added to precipitate the product, followed by filtration. (ii) Ni[P(OCH3)3]jZ+[B(CgHj)1-]*. (iii) Ni[P(OCH3)3]j2'(PF6-)2. (iv) Ni[P(OCH3)3]j2'(~~F6-)2. (v) Ni[P(OCH:)3]s2'(SbF6-)2. (vi) Ni[P(OCH3)3]52+SiFs2-.NiSiF6(0.9 g, 0.0045 mol) was added to 25 ml of methanol in a 125-ml erlenmeyer flask followed by 4.5 g (0.036 mol) of P(OCH3)3. The solids went into solution after stirring for 20 mini the solution was stripped to an orange oil and used in this form for the nmr studies. (vii) Ni[P(OCH)3CCi3]52+(BF(-)2. A procedure analogous to (i) w ~ employed s for this compound and for compound viii using ethanol as the solvent. (viii) Ni[P(OCHz)3CC2H5J62+(BFa-)2. (ix) Ni[P(OCHCH2)3]a2+(BF~-)2. The 2,8,9-trioxa-l-phosphaadamantane complex was prepared iiz situ. To 0.25 ml of CH&N in a IO-" nmr tube was added 50 mg of Ni(BF&.6H20 and 166 mg of P(OCHCH& The volume was raised to 4 ml by adding CHClF2 at Dry Ice temperature and the solution was shaken to ensure complete mixing. V. Palladium Complexes. The precursor for all PdLj2+ preparations was commercial grade PdCI,. (i) Pd[P(OCH3)3]52+[B(C~H5)I-]2. A 125-ml erlenmeyer flask was charged with 50 ml of methanol and palladium chloride (0.88 g, 0.0049 mol) under nitrogen. Trimethyl phosphite (6.80 g, 0.054 mol) was added and stirring was continued until all solids went into solution. Sodium tetraphenylborate (6.84 g, 0.02 mol) was dissolved in a minimum amount of methanol and added slowly to the solution to precipitate white solids. After the addition was com-

plete, stirring was continued for an additional 10 min. The product was then collected on a medium-fritted disk funnel and dried it! cacuo for 2 hr. (ii) Pd[P(OC2H5)3]5Z+[B(C6H&]2.The procedure was analogous to (i) using ethanol as solvent and refluxing prior to precipitation. (iii) Pd[P(OCH2)3CCH3]sZ+[B(C6H~)(-](. The preparation was analogous to (ii) using T H F as solvent and recrystallizing from nitromethane-ether. VI. Platinum Complexes. The precursor for all PtLj2+preparations was commercial grade PtC12. (i) Pt[P(OCH3)3]j2+[B(C6Hj)a-!:.A 125-ml erlenmeyer flask was charged with 50 ml of methanol and platinum chloride (1.32 g, 0.0049 mol). Trimethyl phosphite (6.80 g, 0.054 mol) was added, and the mixture was stirred until all went into solution. Sodium tetraphenylborate (6.84 g, 0.02 mol), dissolved in a minimum amount of methanol, was added dropwise to the solution giving a white precipitate. After the addition was complete, stirring was continued for 10 min. The product was then isolated on a mediumfritted disk funnel and dried itz cacuo at room temperature for 2 hr. The crystals used for the X-ray measurements described in the text were obtained by slow evaporation from methylene chloride in the presence of a slight excess of ligand. A similar procedure was adoped to obtain crystals of Pt[P(OCH3):]j2'(PF6-)2. (ii) Pt[P(OC?H,)3]52+[B(CBHj)a-]2. The preparation was similar to (i); the reaction was carried out in refluxing methanol, the initial precipitate was dissolved in CH2C12,and crystals were obtained on addition of ethanol. (iii) Pt[P(OCH*)3CCHs]j2'[B(CsH5)1-]*. The procedure precisely parallels (ii) with the substitution of T H F for ethanol as solvent.

Acknowledgments. We would like to thank Mr. M. A. Cushing for the preparation of most of the complexes, Messrs. G. Watunya and J. M. White for obtaining some of the nmr spectra, Dr. L. W. Gosser for advice with the Co(C*Hl2)(C8H13)preparation, Dr. L. J. Guggenberger foi the X-ray measurements, and Professor John Verkade for kindly supplying a sample of the phosporadamantane ligand.

Meakin, Jesson

Stereochemical Rigidity in ML5 Complexes

Stereochemical rigidity in ML5 complexes. II. Preparations and

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